1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor, and particularly to an image forming apparatus that form color images on a recording sheet based on image information.
2. Description of the Related Art
Some electrophotographic color image forming apparatuses use a method that there are a plurality of image forming units, and images of different colors are successively transferred onto a transfer belt. For this type of image forming apparatuses, there is known a technique to correct for color registration. Specifically, the image forming apparatus forms color registration patterns on the transfer belt, reads the color registration patterns using a sensor, and calculates color registration error. Then, the image forming apparatus controls the times at which color images are formed based on the calculated registration error (Japanese Laid-Open Patent Publication (Kokai) No. 2004-69908).
Also, there is known techniques to control meandering of the transfer belt in a belt width direction. Specifically, the image forming apparatus detects the position of an edge of the transfer belt in the belt width direction using a sensor, and controls the tilt of one roller among steering rollers supporting the transfer belt so as to correct for meandering (Japanese Laid-Open Patent Publication (Kokai) No. 2000-34031). According to Japanese Laid-Open Patent Publication (Kokai) No. 2000-34031, in order to detect the position of the belt edge in the belt width direction, a sensor is placed at one position on one side of the belt in the belt width direction to continuously detect position variations of the transfer belt.
In the method that controls meandering of the transfer belt using the steering roller as described in Japanese Laid-Open Patent Publication (Kokai) No. 2000-34031, the transfer belt may become unstable due to meandering being uncontrollable at the activation of a roller driving motor or immediately after attachment or removal of a secondary transfer roller or a primary transfer roller. In a case where color images are formed when meandering occurs, image forming positions of colors do not match in a main scanning direction (the belt width direction of the transfer belt, that is, the direction in which the transfer belt meanders), and as a result, color-shifted images are formed.
Moreover, in a case where a color registration correction described in Japanese Laid-Open Patent Publication (Kokai) No. 2004-69908 is performed when the transfer belt meanders, color registration correction is performed based on detected color registration error caused by transitional meandering of the transfer belt. Under normal conditions, in order to reduce downtime, the color registration correction is not performed unless a predetermined number of prints have been made or a predetermined time period has elapsed. As a result, if color registration correction is performed based on an erroneous detection result mentioned above, color-shifted images are continuously output even for a short time period until the next color registration correction is performed.